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Media used in the detection and enumeration of Staphylococcus aureus
Abstract:
Over the years a range of selective and diagnostic media have been developed to assist in the detection and enumeration of Staphylococcus aureus in routine food surveillance programmes and food poisoning investigations and these are reviewed here. Baird-Parker agar remains, however, the medium of choice for direct plating and enumeration of S. aureus in both Europe and the US. This paper also reports on a comparison of the productivity and selectivity of trypticase soy broth with 10% NaCl and 1% sodium pyruvate (PTSBS) with trypticase soy broth (TSB) for the isolation of S. aureus. Using three strains of S. aureus and a strain of S. hyicus the productivity ratio of PTSBS to TSBS ranged from -0.17 to 0.57. In the recovery of heat-injured cells of S. aureus, PTSBS offered little or no improvement over TSB with sodium chloride but no pyruvate in a three-tube MPN assessment. Of the seven other bacterial cultures tested, none grew well on the PTSBS. Selectivity ratios of 4.4-7 were found.
Insights
Baird-Parker agar is the preferred medium for detecting Staphylococcus aureus in food. A modified trypticase soy broth (PTSBS) showed limited improvement for isolating S. aureus, especially heat-injured cells.
Area of Science:
- Microbiology
- Food Safety
- Bacteriology
Background:
- Selective and diagnostic media are crucial for detecting Staphylococcus aureus in food surveillance and poisoning investigations.
- Baird-Parker agar is the established standard for S. aureus enumeration in Europe and the US.
Purpose of the Study:
- To compare the efficacy of trypticase soy broth with 10% NaCl and 1% sodium pyruvate (PTSBS) against standard trypticase soy broth (TSB) for S. aureus isolation.
- To evaluate the productivity and selectivity of PTSBS, particularly for heat-injured S. aureus cells.
Main Methods:
- Comparison of PTSBS and TSB for S. aureus isolation using multiple strains.
- Assessment of recovery for heat-injured S. aureus cells via a three-tube most probable number (MPN) method.
- Testing the growth of seven other bacterial cultures on PTSBS to determine selectivity.
Main Results:
- The productivity ratio of PTSBS to TSB for S. aureus ranged from -0.17 to 0.57.
- PTSBS provided minimal to no advantage over TSB for recovering heat-injured S. aureus.
- PTSBS demonstrated good selectivity, with minimal growth of other tested bacterial species (selectivity ratios of 4.4-7).
Conclusions:
- While PTSBS shows good selectivity, it offers limited improvement over standard TSB for S. aureus isolation, particularly for stressed cells.
- Baird-Parker agar remains the gold standard for routine S. aureus detection and enumeration in food safety applications.